- Email: [email protected]
metal oxide multilayered thin films for low-voltage varistors
Pergamon PII: S0042-207X(98)00293-0
Vacuum/volume 51/number 4/pages 719 to 722/1998 ã 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042-207X/98 Sl - see front matter
Preparation of zinc oxide/metal oxide multilayered thin films for low-voltage varistors N Horio,a* M Hiramatsu,a M Nawata,a K Imaedab and T Toriib, aDepartment of Electrical and Electronic Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku Nagoya 468, Japan, and bNGK Spark Plug Co., Ltd., 14-18 Takatsuji-cho, Mizuho, Nagoya 467, Japan
Zinc oxide/metal oxide multilayered composite thin film varistors have been fabricated by the radio-frequency (RF, 13.56 MHz) sputtering method and the electrical properties of the films were investigated. After the Au electrode was formed on a quartz substrate by vacuum evaporation, zinc oxide (ZnO) layer was deposited by the RF sputtering with ZnO target at room temperature, Ar partial pressure of 2.5 Pa, O2 partial pressure of 2.5 Pa and RF power of 50 W. Praseodymium oxide (Pr6O11) layer was formed on the ZnO layer by the RF sputtering with Pr6O11 target at room temperature, Ar pressure of 5 Pa and RF power of 80 W. The upper Au electrode was formed on the Pr6O11 layer by vacuum evaporation. The conduction mechanism of the ZnO/Pr6O11 double-layered thin film varistor was discussed on the basis of the measurements of voltage-current (V-I) and capacitance-voltage (C-V) characteristics and thermally stimulated current (TSC). From the results of V-I and C-V measurements, it was found that a depletion region was formed in the ZnO layer close to the interface between ZnO and Pr6O11 layers. ã 1998 Elsevier Science Ltd. All rights reserved
Introduction Zinc oxide (ZnO) varistors with several additives of heavymetal oxides such as Bi2O3, Pr6O11, Sb2O3, CoO, MnO and Cr2O3 are widely used as transient surge suppression for protecting electronic circuits against high abnormal voltage surges. They exhibit highly nonlinear voltage-current (V-I) characteristics. In particular, transient surge suppression for low-voltage electronic circuits has attracted attention because of advances of VLSI electronics. Therefore, varistors with highly nonlinear V-I characteristics at applied voltages below 5 V are necessary. For the low-voltage application of varistors, devices require thin ®lm structures (thickness below 10 s mm).1 The highly nonlinear V-I characteristics of the varistors are considered to originate in grain boundaries.2 In the actual ZnO bulk varistors, however, it is dicult to understand the nonlinear V-I characteristics, because the grain boundaries in the bulk varistors are connected in a complex manner, both in series and parallel. The purposes of the present study are to clarify the conduction mechanism of ZnO varistors and to develop the ZnObased thin ®lm varistors for low-voltage application. In this work, as a model of a single grain boundary in a ZnO ceramic varistor, ZnO/praseodymium oxide (Pr6O11) double-layered thin ®lm varistors have been fabricated by the radio-frequency *To whom all correspondence should be addressed
(RF) sputtering method and the electrical properties of the ®lms were investigated. The conduction mechanism of the ZnO/Pr6O11 double-layered thin ®lm varistors was discussed on the basis of the measurements of V-I and capacitance-voltage (C-V) characteristics and thermally stimulated current (TSC).
Experimental The procedure for the preparation of the ZnO/Pr6O11 doublelayered ®lm sample is as follows. After the formation of Au electrode on a quartz substrate by vacuum evaporation, ZnO layer was deposited by the RF (13.56 MHz) sputtering with ZnO target at room temperature, Ar partial pressure of 2.5 Pa, O2 partial pressure of 2.5 Pa and RF power of 50 W. Pr6O11 layer was formed on the ZnO layer by the RF sputtering with Pr6O11 target at room temperature, Ar pressure of 5 Pa and RF power of 80 W. The upper Au electrode was formed on the Pr6O11 layer by vacuum evaporation. The area of thin ®lm varistor sample was 7.3 mm2. The V-I characteristics of ZnO/Pr6O11 double-layered thin ®lm samples were surveyed with a commercial DC source measuring unit in vacuum. The C-V characteristics of the ZnO/Pr6O11 samples were measured at a temperature of 508C using a commercial impedance analyzer with applying an AC voltage of 10 mV at 5 kHz. 719
N Horio et al: Preparation of zinc oxide/metal oxide multilayered films for low-voltage varistors Results and discussion
Figure 1. Schematic sequences of temperature and bias voltage for the measurement of TSC spectrum.
Figure 1 shows the schematic sequences of temperature and bias voltage for the measurement of TSC spectrum of ZnO/ Pr6O11 samples. After the preheating up to 1008C, the sample temperature was kept constant at the bias temperature Tb (30± 808C) in the bias period tb (10 min), while applying the bias voltage Vb (ÿ1 to 20 V) for the formation of space charges in the sample. The sample was cooled down rapidly from Tb to ÿ808C in order to trap carriers. Then the sample was heated continuously at a rate of 48C/min under short-circuited condition and the TSC spectrum which was due to the release of trapped carriers or the depolarization of oriented dipoles was obtained.
Figure 2. Typical V-I characteristics of the ZnO (600 nm)/Pr6O11 (400 nm) double-layered thin film sample measured at a sample temperature of 408C. ZnO[+] represents the reverse bias, and ZnO[ÿ] forward bias.
720
Figure 2 shows the typical V-I characteristics of the ZnO (600 nm)/Pr6O11 (400 nm) double-layered thin ®lm measured at a sample temperature of 408C. The non-ohmic V-I behavior was observed. It should be noted that the V-I characteristic of the double-layered ®lm when the ZnO layer was positively biased (reverse bias, ZnO[+]) showed the predominant nonlinear behavior (a varistor characteristic). In this case, the nonlinear coecient a and the varistor voltage of the ZnO/Pr6O11 double-layered thin ®lm varistor were 10 and 20 V, respectively. When the ZnO layer was negatively biased (forward bias, ZnO[ÿ]), on the other hand, the V-I characteristic was roughly exponential. The V-I characteristics of ZnO or Pr6O11 thin ®lms were also measured. The noticeable nonlinear behavior was not observed in the single ®lm of ZnO or Pr6O11. Therefore, the nonlinear V-I behavior of ZnO/Pr6O11 doublelayered thin ®lm sample is considered to originate in the interface between the ZnO and Pr6O11 layers. The resistivities of the ZnO and Pr6O11 thin ®lms were on the order of 106± 107 O cm. On the other hand, the resistivity of the ZnO/Pr6O11 thin ®lm sample was on the order of 1010 O cm. From these results, it is suggested that a depletion region was formed in the ZnO layer near the interface between the ZnO and Pr6O11 layers. ZnO is an n-type oxygen-defect semiconductor. Then, resultant excess Zn ions act as donors. Depletion, namely, the decrease of donors in the ZnO layer near the interface is considered to be due to the diusion of oxygen atoms into ZnO layer from the Pr6O11 layer through the interface at the early stage of deposition of Pr6O11 ®lm onto the ZnO surface using RF sputtering, resulting in the stoichiometry of ZnO layer near the interface. The breakdown mechanism for the reverse bias is considered to be the electron tunneling process.2 Figure 3 shows the typical C-V behavior for the ZnO/ Pr6O11 double-layered thin ®lm sample, together with the plot
Figure 3. Typical C-V behavior for the ZnO/Pr6O11 double-layered thin film sample. ZnO layer was positively biased.
N Horio et al: Preparation of zinc oxide/metal oxide multilayered films for low-voltage varistors
Figure 4. Depth profile of Nd in the ZnO layer from the interface obtained from Figure 3.
of 1/C2 against V. In Figure 3, the ZnO layer was positively biased. The capacitance due to the depletion region in the ZnO layer near the interface decreased with the increase of applied DC voltage. If 1/C2 is plotted as a function of V, the slope of the curve is expressed by3
@ 1=c2 =@ V 2=EqNd W ,
1
where W is the width of the depletion region in the ZnO layer from the interface, which is equal to the spacing of paralellplate capacitor, Nd(W) is the donor density at W, E is the dielectric constant of ZnO (i.e., 8.5E0) and q is the electronic charge. The plot of 1/C2 against V shown in Figure 3 was not linear, which suggested that the junction of ZnO/Pr6O11 was a Schottky junction with a non-uniform donor concentration in the ZnO layer near the interface. The slope of the plot @(1/ C2)/@V yields Nd(W) at a voltage V using Equation (1). Simultaneously, W is obtained from the measured capacitance at V for a parallel-plate capacitor where the spacing between two plates represents the depletion layer width. Figure 4 shows
Figure 5. Thermally stimulated current (TSC) spectra of ZnO/Pr6O11 double-layered thin film sample as a function of sample temperature. Closed circles represent the data for the reverse bias voltage Vb at 10 V, and open circles the data for the forward bias voltage Vb at 1 V.
Figure 6. Dependence of the peak currents at P1 and P2 peaks in the TSC spectra on the bias temperature Tb.
the depth pro®le of Nd in the ZnO layer from the interface obtained from Figure 3. It was found that the distribution of donors in the ZnO layer near the interface was not uniform and there existed a depletion region. The width of the depletion region without bias voltage was estimated to be 190 nm, and the donor density at Wr 190 nm was approximately 1.6 1018 cmÿ3. TSC measurement was carried out according to the procedure shown in Figure 1. Figure 5 shows the example of TSC spectra of the ZnO/Pr6O11 double-layered thin ®lm sample as a function of the sample temperature. In Figure 5, the bias voltages Vb for the reverse bias (ZnO[+]) and the forward bias (ZnO[ÿ]) were 10 and 1 V, respectively. In both cases, the stimulated currents ¯owed in opposite directions to the applied bias voltage. When the ZnO layer was positively biased (ZnO[+]) before measuring the stimulated current, two TSC peaks, P1 and P2, were observed at sample temperatures around ÿ60 and +508C, respectively. On the other hand,
Figure 7. Arrhenius plot of ln I(T) against 1/T, where I(T) was the thermally stimulated current at rising phase of the TSC form around P1 peak.
721
N Horio et al: Preparation of zinc oxide/metal oxide multilayered films for low-voltage varistors when the ZnO layer was negatively biased (ZnO[ÿ]) beforehand, only one TSC peak P3 was observed around +508C. The eects of bias voltage Vb and bias temperature Tb on the TSC spectra of the ZnO/Pr6O11 double-layered thin ®lm sample were investigated. Figure 6 shows one of the examples of results for TSC measurements, which indicates the dependence of the peak currents at P1 and P2 peaks in the TSC spectra on the bias temperature Tb. The peak current at P1 was almost constant in the Tb region of 30±808C. On the other hand, the peak current at P2 increased with the increase of Tb. Peak current at P2 increased as well with the increase of bias voltage Vb up to 15 V. The plots of the peak currents at P3 against Tb and Vb showed the similar behaviors as those at P2 peak. From these results, the P2 and P3 peaks of the TSC spectra are attributed to the migration of excess Zn ions in the ZnO layer. The P1 peak of the TSC spectrum is considered to be due to the release of electrons trapped by the shallow level in the Pr6O11 layer near the interface. Figure 7 shows the Arrhenius plot of ln I(T) against 1/T, where I(T) was the thermally stimulated current at rising phase of the TSC form around P1 peak. As shown in Figure 7, a linear relation was observed. From the slope of the plot of ln I(T) against 1/T, the thermal emission activation energy of this shallow electron trap was estimated to be 0.01 eV. Recently laser ablation technique has been used successfully to prepare ZnO transparent conducting ®lms. ZnO ®lms prepared by excimer laser ablation exhibited better crystallinity and higher carrier concentration than those formed by sputtering method.4 Therefore, laser ablation technique will be presumably applicable to the preparation of ZnO-based thin ®lm varistors with improved electrical properties such as nonlinear coecient and varistor voltage.
722
Conclusion ZnO/Pr6O11 double-layered thin ®lm varistors have been fabricated by the RF sputtering method, and the electrical properties of the ®lms were examined. The nonlinear coecient a and the varistor voltage of the ZnO/Pr6O11 double-layered thin ®lm varistor were 10 and 20 V, respectively. The conduction mechanism of the ZnO/Pr6O11 thin ®lm varistors was discussed on the basis of the results of V-I, C-V and TSC measurements. It was found that a depletion region was formed in the ZnO layer close to the interface between ZnO and Pr6O11 layers.
Acknowledgements The authors would like to thank Professor Hideaki Kawamura of Meijo University for helpful discussion on this work. The authors are grateful to NIHON KOUATSU ELECTRIC (NKE) CO., Ltd. for supplying the ZnO and Pr6O11 sputtering targets.
References 1. Yano, Y., Shirakawa, Y. and Morooka, H., J. Ceramic Soc. Jpn., 1992, 100, 547. 2. Levinson, L. M. and Philipp, H. R., Zinc Oxide VaristorsÐA Review. Bull. American Ceramic Soc., 1986, 65, 639. 3. Sze, S.M., Semiconductor Devices, Physics and Technology, John Wiley & Sons, New York, 1985. 4. Hiramatsu, M., Imaeda, K., Horio, N. and Nawata, M., (submitted) J. Vac. Sci. Technol. A.
Vacuum/volume 51/number 4/pages 719 to 722/1998 ã 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042-207X/98 Sl - see front matter
Preparation of zinc oxide/metal oxide multilayered thin films for low-voltage varistors N Horio,a* M Hiramatsu,a M Nawata,a K Imaedab and T Toriib, aDepartment of Electrical and Electronic Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku Nagoya 468, Japan, and bNGK Spark Plug Co., Ltd., 14-18 Takatsuji-cho, Mizuho, Nagoya 467, Japan
Zinc oxide/metal oxide multilayered composite thin film varistors have been fabricated by the radio-frequency (RF, 13.56 MHz) sputtering method and the electrical properties of the films were investigated. After the Au electrode was formed on a quartz substrate by vacuum evaporation, zinc oxide (ZnO) layer was deposited by the RF sputtering with ZnO target at room temperature, Ar partial pressure of 2.5 Pa, O2 partial pressure of 2.5 Pa and RF power of 50 W. Praseodymium oxide (Pr6O11) layer was formed on the ZnO layer by the RF sputtering with Pr6O11 target at room temperature, Ar pressure of 5 Pa and RF power of 80 W. The upper Au electrode was formed on the Pr6O11 layer by vacuum evaporation. The conduction mechanism of the ZnO/Pr6O11 double-layered thin film varistor was discussed on the basis of the measurements of voltage-current (V-I) and capacitance-voltage (C-V) characteristics and thermally stimulated current (TSC). From the results of V-I and C-V measurements, it was found that a depletion region was formed in the ZnO layer close to the interface between ZnO and Pr6O11 layers. ã 1998 Elsevier Science Ltd. All rights reserved
Introduction Zinc oxide (ZnO) varistors with several additives of heavymetal oxides such as Bi2O3, Pr6O11, Sb2O3, CoO, MnO and Cr2O3 are widely used as transient surge suppression for protecting electronic circuits against high abnormal voltage surges. They exhibit highly nonlinear voltage-current (V-I) characteristics. In particular, transient surge suppression for low-voltage electronic circuits has attracted attention because of advances of VLSI electronics. Therefore, varistors with highly nonlinear V-I characteristics at applied voltages below 5 V are necessary. For the low-voltage application of varistors, devices require thin ®lm structures (thickness below 10 s mm).1 The highly nonlinear V-I characteristics of the varistors are considered to originate in grain boundaries.2 In the actual ZnO bulk varistors, however, it is dicult to understand the nonlinear V-I characteristics, because the grain boundaries in the bulk varistors are connected in a complex manner, both in series and parallel. The purposes of the present study are to clarify the conduction mechanism of ZnO varistors and to develop the ZnObased thin ®lm varistors for low-voltage application. In this work, as a model of a single grain boundary in a ZnO ceramic varistor, ZnO/praseodymium oxide (Pr6O11) double-layered thin ®lm varistors have been fabricated by the radio-frequency *To whom all correspondence should be addressed
(RF) sputtering method and the electrical properties of the ®lms were investigated. The conduction mechanism of the ZnO/Pr6O11 double-layered thin ®lm varistors was discussed on the basis of the measurements of V-I and capacitance-voltage (C-V) characteristics and thermally stimulated current (TSC).
Experimental The procedure for the preparation of the ZnO/Pr6O11 doublelayered ®lm sample is as follows. After the formation of Au electrode on a quartz substrate by vacuum evaporation, ZnO layer was deposited by the RF (13.56 MHz) sputtering with ZnO target at room temperature, Ar partial pressure of 2.5 Pa, O2 partial pressure of 2.5 Pa and RF power of 50 W. Pr6O11 layer was formed on the ZnO layer by the RF sputtering with Pr6O11 target at room temperature, Ar pressure of 5 Pa and RF power of 80 W. The upper Au electrode was formed on the Pr6O11 layer by vacuum evaporation. The area of thin ®lm varistor sample was 7.3 mm2. The V-I characteristics of ZnO/Pr6O11 double-layered thin ®lm samples were surveyed with a commercial DC source measuring unit in vacuum. The C-V characteristics of the ZnO/Pr6O11 samples were measured at a temperature of 508C using a commercial impedance analyzer with applying an AC voltage of 10 mV at 5 kHz. 719
N Horio et al: Preparation of zinc oxide/metal oxide multilayered films for low-voltage varistors Results and discussion
Figure 1. Schematic sequences of temperature and bias voltage for the measurement of TSC spectrum.
Figure 1 shows the schematic sequences of temperature and bias voltage for the measurement of TSC spectrum of ZnO/ Pr6O11 samples. After the preheating up to 1008C, the sample temperature was kept constant at the bias temperature Tb (30± 808C) in the bias period tb (10 min), while applying the bias voltage Vb (ÿ1 to 20 V) for the formation of space charges in the sample. The sample was cooled down rapidly from Tb to ÿ808C in order to trap carriers. Then the sample was heated continuously at a rate of 48C/min under short-circuited condition and the TSC spectrum which was due to the release of trapped carriers or the depolarization of oriented dipoles was obtained.
Figure 2. Typical V-I characteristics of the ZnO (600 nm)/Pr6O11 (400 nm) double-layered thin film sample measured at a sample temperature of 408C. ZnO[+] represents the reverse bias, and ZnO[ÿ] forward bias.
720
Figure 2 shows the typical V-I characteristics of the ZnO (600 nm)/Pr6O11 (400 nm) double-layered thin ®lm measured at a sample temperature of 408C. The non-ohmic V-I behavior was observed. It should be noted that the V-I characteristic of the double-layered ®lm when the ZnO layer was positively biased (reverse bias, ZnO[+]) showed the predominant nonlinear behavior (a varistor characteristic). In this case, the nonlinear coecient a and the varistor voltage of the ZnO/Pr6O11 double-layered thin ®lm varistor were 10 and 20 V, respectively. When the ZnO layer was negatively biased (forward bias, ZnO[ÿ]), on the other hand, the V-I characteristic was roughly exponential. The V-I characteristics of ZnO or Pr6O11 thin ®lms were also measured. The noticeable nonlinear behavior was not observed in the single ®lm of ZnO or Pr6O11. Therefore, the nonlinear V-I behavior of ZnO/Pr6O11 doublelayered thin ®lm sample is considered to originate in the interface between the ZnO and Pr6O11 layers. The resistivities of the ZnO and Pr6O11 thin ®lms were on the order of 106± 107 O cm. On the other hand, the resistivity of the ZnO/Pr6O11 thin ®lm sample was on the order of 1010 O cm. From these results, it is suggested that a depletion region was formed in the ZnO layer near the interface between the ZnO and Pr6O11 layers. ZnO is an n-type oxygen-defect semiconductor. Then, resultant excess Zn ions act as donors. Depletion, namely, the decrease of donors in the ZnO layer near the interface is considered to be due to the diusion of oxygen atoms into ZnO layer from the Pr6O11 layer through the interface at the early stage of deposition of Pr6O11 ®lm onto the ZnO surface using RF sputtering, resulting in the stoichiometry of ZnO layer near the interface. The breakdown mechanism for the reverse bias is considered to be the electron tunneling process.2 Figure 3 shows the typical C-V behavior for the ZnO/ Pr6O11 double-layered thin ®lm sample, together with the plot
Figure 3. Typical C-V behavior for the ZnO/Pr6O11 double-layered thin film sample. ZnO layer was positively biased.
N Horio et al: Preparation of zinc oxide/metal oxide multilayered films for low-voltage varistors
Figure 4. Depth profile of Nd in the ZnO layer from the interface obtained from Figure 3.
of 1/C2 against V. In Figure 3, the ZnO layer was positively biased. The capacitance due to the depletion region in the ZnO layer near the interface decreased with the increase of applied DC voltage. If 1/C2 is plotted as a function of V, the slope of the curve is expressed by3
@ 1=c2 =@ V 2=EqNd W ,
1
where W is the width of the depletion region in the ZnO layer from the interface, which is equal to the spacing of paralellplate capacitor, Nd(W) is the donor density at W, E is the dielectric constant of ZnO (i.e., 8.5E0) and q is the electronic charge. The plot of 1/C2 against V shown in Figure 3 was not linear, which suggested that the junction of ZnO/Pr6O11 was a Schottky junction with a non-uniform donor concentration in the ZnO layer near the interface. The slope of the plot @(1/ C2)/@V yields Nd(W) at a voltage V using Equation (1). Simultaneously, W is obtained from the measured capacitance at V for a parallel-plate capacitor where the spacing between two plates represents the depletion layer width. Figure 4 shows
Figure 5. Thermally stimulated current (TSC) spectra of ZnO/Pr6O11 double-layered thin film sample as a function of sample temperature. Closed circles represent the data for the reverse bias voltage Vb at 10 V, and open circles the data for the forward bias voltage Vb at 1 V.
Figure 6. Dependence of the peak currents at P1 and P2 peaks in the TSC spectra on the bias temperature Tb.
the depth pro®le of Nd in the ZnO layer from the interface obtained from Figure 3. It was found that the distribution of donors in the ZnO layer near the interface was not uniform and there existed a depletion region. The width of the depletion region without bias voltage was estimated to be 190 nm, and the donor density at Wr 190 nm was approximately 1.6 1018 cmÿ3. TSC measurement was carried out according to the procedure shown in Figure 1. Figure 5 shows the example of TSC spectra of the ZnO/Pr6O11 double-layered thin ®lm sample as a function of the sample temperature. In Figure 5, the bias voltages Vb for the reverse bias (ZnO[+]) and the forward bias (ZnO[ÿ]) were 10 and 1 V, respectively. In both cases, the stimulated currents ¯owed in opposite directions to the applied bias voltage. When the ZnO layer was positively biased (ZnO[+]) before measuring the stimulated current, two TSC peaks, P1 and P2, were observed at sample temperatures around ÿ60 and +508C, respectively. On the other hand,
Figure 7. Arrhenius plot of ln I(T) against 1/T, where I(T) was the thermally stimulated current at rising phase of the TSC form around P1 peak.
721
N Horio et al: Preparation of zinc oxide/metal oxide multilayered films for low-voltage varistors when the ZnO layer was negatively biased (ZnO[ÿ]) beforehand, only one TSC peak P3 was observed around +508C. The eects of bias voltage Vb and bias temperature Tb on the TSC spectra of the ZnO/Pr6O11 double-layered thin ®lm sample were investigated. Figure 6 shows one of the examples of results for TSC measurements, which indicates the dependence of the peak currents at P1 and P2 peaks in the TSC spectra on the bias temperature Tb. The peak current at P1 was almost constant in the Tb region of 30±808C. On the other hand, the peak current at P2 increased with the increase of Tb. Peak current at P2 increased as well with the increase of bias voltage Vb up to 15 V. The plots of the peak currents at P3 against Tb and Vb showed the similar behaviors as those at P2 peak. From these results, the P2 and P3 peaks of the TSC spectra are attributed to the migration of excess Zn ions in the ZnO layer. The P1 peak of the TSC spectrum is considered to be due to the release of electrons trapped by the shallow level in the Pr6O11 layer near the interface. Figure 7 shows the Arrhenius plot of ln I(T) against 1/T, where I(T) was the thermally stimulated current at rising phase of the TSC form around P1 peak. As shown in Figure 7, a linear relation was observed. From the slope of the plot of ln I(T) against 1/T, the thermal emission activation energy of this shallow electron trap was estimated to be 0.01 eV. Recently laser ablation technique has been used successfully to prepare ZnO transparent conducting ®lms. ZnO ®lms prepared by excimer laser ablation exhibited better crystallinity and higher carrier concentration than those formed by sputtering method.4 Therefore, laser ablation technique will be presumably applicable to the preparation of ZnO-based thin ®lm varistors with improved electrical properties such as nonlinear coecient and varistor voltage.
722
Conclusion ZnO/Pr6O11 double-layered thin ®lm varistors have been fabricated by the RF sputtering method, and the electrical properties of the ®lms were examined. The nonlinear coecient a and the varistor voltage of the ZnO/Pr6O11 double-layered thin ®lm varistor were 10 and 20 V, respectively. The conduction mechanism of the ZnO/Pr6O11 thin ®lm varistors was discussed on the basis of the results of V-I, C-V and TSC measurements. It was found that a depletion region was formed in the ZnO layer close to the interface between ZnO and Pr6O11 layers.
Acknowledgements The authors would like to thank Professor Hideaki Kawamura of Meijo University for helpful discussion on this work. The authors are grateful to NIHON KOUATSU ELECTRIC (NKE) CO., Ltd. for supplying the ZnO and Pr6O11 sputtering targets.
References 1. Yano, Y., Shirakawa, Y. and Morooka, H., J. Ceramic Soc. Jpn., 1992, 100, 547. 2. Levinson, L. M. and Philipp, H. R., Zinc Oxide VaristorsÐA Review. Bull. American Ceramic Soc., 1986, 65, 639. 3. Sze, S.M., Semiconductor Devices, Physics and Technology, John Wiley & Sons, New York, 1985. 4. Hiramatsu, M., Imaeda, K., Horio, N. and Nawata, M., (submitted) J. Vac. Sci. Technol. A.